Most oil and gas wells are drilled with drill pipe. After reaching a selected depth, the operator makes up and lowers a string of casing into the well and cements it in place. In another technique, the casing is employed as the drill string while drilling the well. In this technique, a bit and a reamer are located at the lower end of the casing. The drilling rig rotates the string of casing, the drill bit, and the reamer. Drilling fluid is pumped down the casing, which discharges out the bit and flows back up the annulus surrounding the casing. A mud motor can be incorporated with the drill bit and reamer for rotating the drill bit and reamer relative to the casing in response to drilling fluid pumped down the string of casing.
When reaching a desired depth, the operator optionally may cement the drill bit assembly in place. If the well is at total depth, the operator then completes the well by normally running a string of production tubing inside the casing. The operator may also retrieve the drill bit assembly whether or not the casing string is at total depth. If not at total depth, the operator may rerun the drill bit assembly with a new bit or other components of the bottom hole assembly. Retrieving the bottom hole assembly may be done in different manners: with a wire line; circulating drilling fluid from the casing annulus up the string of casing to pump the drill bit assembly up to the surface; and running a string of drill pipe into the string of casing.
If the casing string is at a desired depth and additional casing strings are planned, another technique known in the prior art is to use a bit assembly wherein the center portion is retrievable and the outer portion remains secured to the exterior of the casing shoe and is cemented in the well. A disadvantage is that the outer portion may have expensive cutting and wear resistant elements, such as diamond or tungsten carbide cutting elements, that could be reused. This disadvantage also results wherein the entire cutting structure, including the drill bit and reamer, remains attached to the string of casing and is drilled through after the casing string has been cemented in place.
Another technique is to use pivotal reamer arms on the bit assembly. The reamer arms contain cutting elements for cutting the outer portion of the wellbore. The reamer arms are normally located some distance below the casing shoe attached to the lower end of the string of casing. In that instance, all of the cutting structure is retrieved. Alternately, pivotal reamer arms that are located in slots of the casing shoe are known.
While these various techniques are workable, making a retrievable bit assembly simpler, easier to retrieve, and less expensive would be desirable. These qualities would be particularly advantageous when drilling an upper portion of the well with a string of casing, referred to as surface casing, which often can be done without having to change-out the drill bit.